1. Field
The present disclosure relates to a method for manufacturing a silicon carbide semiconductor device and the silicon carbide semiconductor device, and relates, in particular, to a method for reducing the number of defects of a silicon carbide epitaxial film formed on a silicon carbide substrate.
2. Related Art
Although conventional power semiconductor devices have been formed on a silicon substrate, silicon material has approached a physical limit in performance properties. Attention has focused on silicon carbide (SiC) semiconductor devices, in which a substrate has excellent breakdown electric field strength, holding low power loss capability, and exercising excellent performance in high temperature condition and also in high frequency operation. With regard to the SiC semiconductor device, a device structure is generally fabricated as follows: a SiC epitaxial film is formed on a SiC substrate being a primary substrate with low resistance, and processes such as implanting impurity ions are successively performed on the SiC epitaxial film.
There are many polytypes (crystal polymorphism) such as 2H—SiC, 3C—SiC, 4H—SiC, 6H—SiC, 15R—SiC based upon a periodic structure difference when Si and C are bonded to form SiC. Then, there is a problem that a mismatch can be easily created during crystal growth. Therefore when a SiC single crystal is prepared, different polytypes are inevitably mixed in the crystal, resulting in the existence of multiple crystal defects such as dislocations caused by crystal mismatching. When the SiC epitaxial film is deposited on the SiC substrate, threading screw dislocations and threading edge dislocations may propagate, with just the geometry as they are or with conversion into basal plane dislocations and carrot defects, to the epitaxial film, producing defects in the epitaxial film.
The epitaxial film also has defects created from other causes not depending on the primary substrate. There are defects such as, for example, step bunching which occurs during film deposition and further so-called downfall defects produced when particles are attached to the wafer surface during epitaxial growth.
If a crystal defect may exists in the SiC epitaxial film, the manufactured SiC semiconductor device may cause abnormality in leak current and breakdown voltage failure, resulting in low yield of the product.
Patent Literatures 1 through 5 listed below disclose methods for reducing defects so that the methods are carried out on the SiC substrate surface before forming the epitaxial film. Further, in Patent Literature 6 listed below, a method for reducing defects in the epitaxial film is disclosed so that the SiC epitaxial film formed on the SiC substrate is heated to become up to 1 nm or more in surface roughness Ra of the epitaxial film and then flattened to become less than 0.5 nm in Ra.
Patent Literature 1: Japanese Patent Application LAID-Open No. 2005-311348
Patent Literature 2: JP-A 2006-032655
Patent Literature 3: JP-A 2008-230944
Patent Literature 4: JP-A 2010-182782
Patent Literature 5: WO 2010/090024
Patent Literature 6: JP-A 2008-222509